1. Field of the Invention
The present invention is directed to methods and apparatus for measuring activity of a tracer in a subject in a medical imaging protocol.
2. Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions. Other such functional imaging modalities are known, such as SPECT.
In such functional images, many important pathologies and anatomical structures appear as very high (or low) intensities. For example, a tumour in an FDG-PET image will often appear as a bright region
Kinetic modelling of the imaging data from dynamic PET acquisitions, for example to model one or more biological functions, requires measurement of a blood input function (BIF), the blood pool time activity curve. This BIF can be measured directly from blood sampling, or indirectly from the PET image using a volume of interest (VOI) positioned within a major vessel or the heart. Typically, dynamic PET acquisitions are performed using a single bed position (or fixed field of view, FOV), which providing a major vessel is present within the FOV, allows an image-derived BIF to be measured.
A new imaging protocol developed by Siemens Molecular Imaging allows the acquisition of a whole body (WB) dynamic scan. In this protocol, a whole body is imaged in repeated static-scan-style passes, with each pass representing a single “timeslice” or frame and containing multiple bed positions. The multiple bed position scans are typically taken in sequence (one after another in real time), but combined to form a single timeslice/frame. Each bed position is therefore scanned, non-continuously, multiple times, each for the same number of passes.
Since multiple bed positions are acquired, a single section of a given vessel is not imaged throughout the duration of a scan. As such, measurement of an image-derived BIF from a fixed region is problematic, if not impossible.
Previously considered options to address this issue include:                Avoiding using an image-derived BIF and instead use a blood-sampling-derived BIF. However, the invasiveness of this procedure makes it undesirable for routine use.        A reference region-based kinetic model could be used, which does not require a BIF, but instead a VOI placed in a region where no specific binding/uptake of the radiotracer occurs. Unfortunately, this method is impractical for common tracers such as 18F-FDG where perfused regions lacking specific binding/uptake cannot be identified.        